Aoraki/Mt Cook shrinks by 30m

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New Zealand's tallest peak is a little shorter than previously thought after researchers found its summit had dropped by 30 metres.

Aoraki/Mt Cook is officially listed as 3754 metres above sea level, but new measurements by Otago University researchers revealed it is actually only 3724m at its highest point.

In 1991 there was a massive rock-ice collapse at the summit, lopping 10m off the height of the mountain.

Otago National School of Surveying researcher Dr Pascal Sirguey said the discrepancy between the old height and the new height could be explained by a two-decade long reshaping process following the collapse, affecting the remnant of the original thick ice cap.

"By carefully studying photos taken after the collapse, it appears that there was still a relatively thick ice cap, which was most likely out of balance with the new shape of the summit ridge," Sirguey said.

"As a result the ice cap has been subject to erosion over the past 20 years.

"While the effects of climate change may spring to mind as an explanation, it is probably a case of a simple change in the geomorphology of the mountain."

Despite the shrinkage, Aoraki/Mt Cook is still king of New Zealand's mountains, with the next-tallest, Mt Tasman, lagging behind at 3497m, more than 200m shorter.

To observe Aoraki/Mt Cook's tapu status the expedition spoke with local iwi before the surveying, and the climbers did not step on the summit.

They instead took measurements with surveying equipment while at the top of the ice cap a few metres away and below the true summit.

Land Information New Zealand (LINZ) chief geodesist Graeme Blick said the new measurement will become the official height.

"It's a significant change that the Otago team has recorded and we'll update our data to reflect these findings," he said.

"This means LINZ online data will use this new height and it will be incorporated when we next print hard copy topographic maps for the Aoraki/Mt Cook region."